[OXO cube]

Alan i think the only way remove the rough inner would be to make a 2 part mould , similar to when casting resin bodies and figures
with an inlet hole and breather hole 
most likely the syringe method would be employed in this case 
but could be more expensive due to using more silicone rubber for the moulds
i have found that the inners are easily ground and therefore the roughness is not really noticeable after truing

deffo an option though  for the open wheeler types 

kev